1. Field of the Invention
The present invention relates to an electric power steering system which performs a steering assist operation by transmitting a torque generated by an electric motor to a steering mechanism.
2. Description of Related Art
Electric power steering systems are conventionally utilized which assist a driver""s steering operation by transmitting a torque generated by an electric motor to a steering mechanism. The electric motor is controlled on the basis of a vehicle speed and a steering torque applied to a steering wheel. That is, a target electric current value of the electric motor is set greater as the steering torque increases, and set smaller as the vehicle speed increases. A vehicle speed responsive control operation is performed to variably set the target electric current value of the electric motor in accordance with the vehicle speed, whereby the driver can properly perform the steering operation while feeling a moderate road surface reaction force in any vehicle speed ranges.
However, the vehicle speed responsive control requires an input interface for sampling a signal from a vehicle speed sensor. This is one cause of the cost increase of the electric power steering system.
Since the road surface reaction force varies depending not only on the vehicle speed but also on road surface conditions and vehicle traveling conditions (e.g., slalom traveling and lane change), the vehicle speed responsive control does not always ensure a proper steering assist operation. Where the friction coefficient of a road surface is extremely low, for example, a good steering feeling cannot be provided depending on situation.
It is an object of the present invention to provide an electric power steering system which is capable of properly performing a steering assist operation without the use of vehicle speed information.
The present invention is directed to an electric power steering system which employs an electric motor as a driving source to apply a steering assist force to a steering mechanism. The electric power steering system comprises: an operation degree detecting section for detecting an operation degree of an operation member for steering; a motor current detecting section for detecting an electric current flowing through the electric motor; a steering angle detecting section for detecting a steering angle of the steering mechanism; a reaction force computing section for computing a reaction force from a road surface on the basis of outputs of the operation degree detecting section, the motor current detecting section and the steering angle detecting section; a target value setting section for determining a driving target value for driving the electric motor on the basis of the operation degree detected by the operation degree detecting section and the reaction force computed by the reaction force computing section; and a motor controlling section for controlling the electric motor for driving thereof on the basis of the driving target value determined by the target value setting section and the output of the motor current detecting section.
According to the present invention, the reaction force from the road surface is computed on the basis of the operation degree of the operation member (e.g., a steering torque), the motor current and the steering angle. The driving target value of the electric motor is determined with reference to the road surface reaction force thus computed.
Where the driving target value is determined on the basis of vehicle speed information, no consideration is given to road surface conditions (e.g., friction coefficient and the like). Where the driving target value is determined on the basis of the reaction force from the road surface, on the contrary, a steering assist operation can properly be performed in accordance with the road surface conditions, traveling conditions and the like. This improves a steering feeling.
The operation degree detecting section may be a torque sensor for detecting a steering torque applied to the operation member (e.g., a steering wheel).
Where the steering mechanism is constructed such that a pinion fixed to a steering shaft is meshed with a rack displaceable transversely of a motor vehicle, the steering angle detecting section may be a rotation angle sensor for detecting a rotation angle of the pinion or may be a rack displacement detecting mechanism for detecting a displacement degree of the rack.
The electric power steering system preferably further comprises a road surface reaction force adjusting section for setting a degree of contribution of the reaction force computed by the reaction force computing section to the driving target value to adjust a magnitude of the road surface reaction force to be received by a driver from the operation member.
With this arrangement, a steering feeling can be adjusted to suit a driver""s preference by adjusting the magnitude of the road surface reaction force.
The road surface reaction force adjusting section nay comprise a setting operation member to be operated by the driver, and a filtering section having a band pass characteristic to be adjusted by the operation of the setting operation member for selectively passing outputs of the reaction force computing section. A gain adjusting section for adjusting an output gain of the reaction force computing section may be provided in addition to the filtering section or instead of the filtering section.
The foregoing and other objects, features and effects of the present invention will become more apparent from the following description of the preferred embodiments with reference to the attached drawings.